In most advanced democracies, a large centre-right party competes with a large centre-left party. Of course, the extent to which an electoral system favours large parties – by having high popular-vote thresholds to enter parliament, or through winner-take-all constituencies – affects the degree of political fragmentation. But, by and large, the developed democracies are characterised by competition between large parties on the centre left and centre right. What, then, are true centrists like Mario Monti, Italy’s respected technocratic prime minister, to do?

To be sure, regional and ethnic allegiances play a greater role in some places in Europe – for example, Scotland, Belgium, and Catalonia – but far more so in emerging countries, where political cleavages also reflect specific post-colonial circumstances and often the legacy of single-party rule. Nonetheless, even in ‘emerging market’ democracies, such as Chile, Mexico, South Korea, and India, a left-right cleavage plays an important role – while those who claim the political centre generally remain weak.

The British Liberal Democrats, for example, have tried for decades to become a strong centrist third party, without success. While the political vocabulary in the United States is different, the Democratic Party, since Franklin Roosevelt’s presidency, is indeed a centre-left force, the Republican Party occupies the right, and no other significant party exists.

In France and Germany, there is more fragmentation. Politics is still dominated by a large centre-left party and a large centre-right party, but smaller groups – some claiming the centre and others the right and left extremes – challenge them to various degrees. In some countries, the ‘Greens’ have their own identity, close to the left; but, despite remarkable progress in Germany, they remain unable to reach the electoral size of the large centre-right and centre-left parties.

Variations of this basic structure exist in Spain, Portugal, Greece, Turkey, and the Nordic countries. The situation is particularly interesting in Italy, where Monti, having decided to contest the upcoming general election, has had to position himself on the right (which he signalled by attending a gathering of the leaders of Europe’s centre-right parties). He and former Prime Minister Silvio Berlusconi are now fighting for space on the right, with the centre-left Democrats leading in the polls.

There are at least four differences between centre-right and centre-left approaches to social and economic challenges. The right has greater confidence in markets to allocate resources and provide appropriate incentives; favours private consumption over public goods; is minimally concerned with economic inequality; and tends to be more nationalistic and less optimistic about international cooperation.

The left, by contrast, believes that markets, particularly financial markets, need considerable government regulation and supervision to function well; gives greater weight to public goods (for example, parks, a clean environment, and mass-transit systems); seeks to reduce economic inequality, believing that it undermines democracy and the sense of fairness that is important to well-being; and is more willing to pursue international cooperation as a means to secure peace and provide global public goods, such as climate protection.

When looking at actual economic policies as they have evolved over decades, we see that they always combine centre-right and centre-left elements. Repeated financial crises have tempered even the right’s faith in unregulated markets, while the left has become more realistic and cautious about state planning and bureaucratic processes. Likewise, the choice between privately consumed and publicly consumed ‘goods’ is often blurred, as politicians tend to reinforce citizens’ understandable tendency to demand public goods while rejecting the taxes needed to pay for them.

As income inequality has increased – dramatically in some countries, such as the US – it is moving to the forefront of the debate, reinforcing the traditional political divide. Nonetheless, the centre right and the centre left are arguing about the degree of redistribution, not about the need for some progressivity in taxes and transfers. Both also agree on the need for international co-operation in an increasingly interdependent world, with differences mainly concerning how much effort to spend on it.

So, given that differences in policies as they are implemented have become largely a matter of degree, why do centrist parties remain weak? Why have they failed to unite moderates on both sides of the ideological divide?

One reason is that only a minority of any population is active politically. Active party members hold more ideologically consistent views – and hold them more strongly – than most of those who are politically less engaged, giving activists disproportionate influence in the political process. After all, more nuanced ideas and policy proposals are relatively difficult to propagate effectively enough to generate broad and enthusiastic popular support.

But there also really are fundamental differences in values and economic philosophies, as well as in economic interests, leading to a fairly consistent positioning of voters on the right or left. Disagreement may lead to compromises, but that does not change the underlying differences in starting positions.

It is probably a good thing that structured competition between large centre-right and centre-left parties persists. Such parties can help to integrate the extremes into the political mainstream, while facilitating alternation in power, which is essential to any democracy’s dynamism; a system in which a large centrist party remained permanently in power would be far less desirable. Those, like Monti, who want to mount a challenge from the centre, however personally impressive they may be, have steep obstacles to overcome, and for good reasons.